Thermo-acoustic instabilities in combustion chambers are generated by the interactions between a flame and the combustor acoustics, leading to a resonant coupling. These self-sustained oscillations may be observed in many practical systems such as domestic boilers, industrial furnaces, gas turbines or rocket engines. Although this phenomenon has already been the topic of many investigations, there is yet no generalized robust framework to predict the onset of these self-sustained oscillations and to determine the evolution of the flow variables within the combustor during unstable operation. This work builds on previous models and experiments to improve the description of the response of laminar conical flames to flow perturbations and the prediction of thermoacoustic instability in burners operating with conical flames. In the first part of the manuscript, an extensive review of conical flame dynamics modeling is undertaken and a general framework for the modeling of their Flame Transfer Function (FTF) is presented. The experimental setup and the diagnostics used to characterize their response to flow disturbances are then described. They are used to measure the FTF when the flames are submitted to harmonic flow perturbations. A novel experimental technique is also proposed to control the flow perturbation level at the burner outlet. It enables to modulate the flow with random white noise perturbations and to measure the FTF with a better frequency resolution. Results with this alternative technique compare well with results from the classical method using harmonic signals for small disturbances. Limits of this technique are also highlighted when the perturbation level increases. Different analytical expressions for the FTF of conical flames are derived in the second part of the thesis by progressively introducing more physics into the models. Models based on convected flow disturbances are extended by taking into account the incompressible nature of the perturbed velocity field. It is shown that the prediction of the FTF phase lag of a conical flame is greatly improved and collapses well with measurements. Then, a thorough investigation of the flame base dynamics interacting with the anchoring device is conducted by considering unsteady heat loss from the flame to the burner. This mechanism is shown to drive the motion of the flame base and the flame dynamics at high frequencies. It is also shown that this contribution to the FTF rules the high frequency behavior of the FTF as well as the nonlinear evolution of the FTF when the perturbation level increases. Finally, an analysis is conducted on the dynamics of a single conical flame placed into cylindrical flame tubes featuring different diameters. It is shown that confinement effects need to be taken into account when the burnt gases cannot fully expand. Large differences are observed between FTF measured for different confinement tube diameters. A new dimensionless number is derived to take these effects into account and make all the FTF collapse on a single curve. These different models are then used to model the response of a collection of small conical flames stabilized on a perforated plate. It is shown that by sorting out the different contributing mechanisms to the FTF, the expressions proposed in this work may be combined to capture the main behavior and correct phase lag evolution of these flames in the frequency range of interest for thermo-acoustic instability prediction.

The absorption enhancing ability of chitosan, a linear polysaccharide, is mediated by protonated amino groups on the C-2 position of the molecules that induce interaction with the anionic sites on the cell membranes to subsequently alter tight junction integrity. In neutral and basic environments, such as those found in the small and large intestines, most chitosan molecules will lose their charge and precipitate from solution rendering it ineffective as an absorption enhancer. To increase the solubility of this polymer, methylation of the amino groups on the C- 2 position was proposed. A partially quaternised and water soluble derivative of chitosan, N-trimethyl chitosan chloride (TMC), which exhibits superior solubility in a basic environment compared with other chitosan salts was synthesised and included in a chitosan microbead solid drug delivery system. Two TMC derivatives were synthesised by reductive methylation from high and medium molecular weight Chitoclear™ chitosan respectively. The degree of quaternisation calculated from the 1H-NMR spectra for the medium molecular weight TMC (TMC-M) and the high molecular weight TMC (TMC-H) polymers were 74.7 % and 48.5 % respectively. The mean molecular weights of the synthesised TMC-M and TMC-H polymers were 64 100 g/mole and 233 700 g/mole respectively. The effect of different concentrations TMC-M and TMC-H on chitosan microbeads was studied with results obtained from scanning electron microscopy (SEM), TMC loading capacity and microbead swelling behaviour. After selection of the most suitable TMC concentration, the effect of varying concentration (0.1, 0.2 and 0.5 %) additives on TMC and ibuprofen release was studied. Commonly used modified cellulose gum (Ac-di-sol®(ADS)), sodium starch glycolate (Explotab®(EXP)) and ascorbic acid (AA) were added as disintegrants to different microbead formulations to promote release of both the ibuprofen as model drug and TMC from the beads. It was noticed that the loading (% drug loading capacity) of TMC-M was much lower than that obtained with TMC-H while the inclusion of different additives in varying concentrations did not seem to have a profound influence on the loading of either TMC-M or TMC-H. It was further noticed from the fit factors (f1 and f2) for dissolution profiles of eighteen chitosan microbead variations that the formulation containing TMC-H and 0.5% (w/v) ascorbic acid was the only formulation with a significantly higher ibuprofen and TMC-H release profile compared to all other formulations tested. The chitosan microbead formulation containing 2%(w/v) TMC-H and 0.5 % (w/v) ascorbic acid (H-AA-0.5) was used for in vitro absorption studies through rat intestine in Sweetana-Grass diffusion chambers. Chitosan containing TMC-H (no ascorbic acid) (CHIT-H) only and a plain chitosan microbead (CHIT) formulation was used as control formulations during the in vitro studies. Although the H-AA-0.5 formulation exhibited the highest transport rate for ibuprofen, the mean rate of transport (P app) obtained from the two formulations containing TMCH (CHIT-H and H-AA-0.5) showed no significant difference in the transport rate of ibuprofen. Compared to the CHlT formulation as control, both formulations containing TMC-H exhibited increased ibuprofen transport across in vitro rat jejunum. However, a statistical significant increase in transport was obtained only from the H-AA-0.5 formulation in comparison with the CHlT formulation. It can be concluded that the combination of high molecular weight TMC with a low degree of quaternisation and ascorbic acid (0.5% w/v) in a chitosan microbead lead to a statistical significant increase in the in vitro transport rate of ibuprofen through rat jejunum. / Thesis (Ph.D. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2006.

This thesis describes an experimental investigation of lean blowoff for H2/CO/CH4 mixtures in a swirling combustor. This investigation consisted of three thrusts. The first thrust focused on correlations of the lean blowoff limits of H2/CO/CH4 mixtures under different test conditions. It was found that a classical Damköhler number approach with a diffusion correction could correlate blowoff sensitivities to fuel composition over a range of conditions. The second part of this thesis describes the qualitative flame dynamics near blowoff by systematically characterizing the blowoff phenomenology as a function of hydrogen level in the fuel. These near blowoff dynamics are very complex, and are influenced by both fluid mechanics and chemical kinetics; in particular, the role of thermal expansion across the flame and extinction strain rate were suggested to be critical in describing these influences. The third part of this thesis quantitatively analyzed strain characteristics in the vicinity of the attachment point of stable and near blowoff flames. Surprisingly, it was found that in this shear layer stabilized flame, flow deceleration is the key contributor to flame strain, with flow shear playing a relatively negligible role. Near the premixer exit, due to strong flow deceleration, the flame is negatively strained i.e., compressed. Moving downstream, the strain rate increases towards zero and then becomes positive, where flames are stretched. As the flame moves toward blowoff, holes begin to form in the flame sheet, with a progressively higher probability of occurrence as one moves downstream. It is suggested that new holes form with a more uniform probability, but that this behavior reflects the convection of flame holes downstream by the flow. It has been shown in prior studies, and affirmed in this work, that flames approach blowoff by first passing through a transient phase manifested by local extinction events and the appearance of holes on the flame. A key conclusion of this work is that the onset of this boundary occurs at a nearly constant extinction strain rate. As such, it is suggested that Damköhler number scalings do not describe blowoff itself, but rather the occurrence of this first stage of blowoff. Given the correspondence between this first stage and the actual blowoff event, this explains the success of classical Damköhler number scalings in describing blowoff, such as shown in the first thrust of this thesis. The physics process associated with the actual blowoff event is still unclear and remains a key task for future work.

A new approach was developed to determine the operational stability margin of a laboratory scale combustor. Applying modern and robust techniques and tools from Dynamical System Theory, the approach was based on three basic steps. In the first step, a gray-box thermoacoustical model for the combustor was derived. The second step consisted in applying System Identification techniques to experimental data in order to validate the model and estimate its parameters. The application of these techniques to experimental data under different operating conditions allowed us to determine the functional dependence of the model parameters upon changes in an experimental control parameter. Finally, the third step consisted in using that functional dependence to predict the response of the system at different operating conditions and, ultimately, estimate its operational stability margin. The results indicated that a low-order stochastic non-linear model, including two excited modes, has been identified and the combustor operational stability margin could be estimated by applying a continuation method.

Thesis (M.Sc.) (Hons)-- University of Western Sydney, 2002. / "A thesis submitted in fulfillment of the requirements for the Degree of Master of Science (Honours) in Physics at the University of Western Sydney" "September 2002" "Kin Wa (Karl) Chan of Medical Physics Department of Westmead Hospital and the University of Western Sydney"-- t.p. Bibliography: leaves 100-105.

Thesis (M.Sc.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 225-238.

Thesis (doctoral)--University of Göteborg, 1997. / Added t.p. with thesis statement inserted.

Thesis (J.S.M.)--Stanford University, 2009. / Submitted to the Stanford Program in International Legal Studies at the Stanford Law School, Stanford University. "April 2009." Includes bibliographical references (leaves 78-82). Abstract available online.

Thesis (Ph.D)--Aerospace Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Seitzman, Jerry; Committee Member: Filatyev, Sergei; Committee Member: Jagoda, Jechiel; Committee Member: Lieuwen, Timothy; Committee Member: Shelton, Samuel; Committee Member: Zinn, Ben. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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